Distributed acoustic monitoring is becoming an important method for downhole monitoring of wells, with many potential applications. This technology involves the use of an optical cable having an optical fiber contained within a metal tube, with gels or other substances used to hold the fiber within the tube. As the cable is deformed, the length of the optical fiber is changed, which can be detected through proper optical interrogation of the fiber. Generally, this involves sending short bursts of light down the fiber, and recording the light that is reflected back via Rayleigh backscatter.
In general, in these applications, either incident acoustic pressure or alternatively, the vibration of a surface to which the optical cable is attached, results in the bending of the optical cable. Since the cable is predominately sensitive to longitudinal deformation, care must be taken to ensure that the bending of the cable actually produces a change in the length of the optical fiber. Attempts to increase the sensitivity of the fiber have placed the fiber away from the center of the cable, and nearer to the outside of the cable, as this increases the strain that occurs in the fiber. However, for some applications, such as vertical seismic profiling (“VSP”), even greater sensitivity of the fiber may be needed.
Thus, the art would be receptive to alternative devices and methods for improving the sensitivity of optical fibers in optical cables.